New frequency bands for different technologies are continuously being standardized. Various internal and regional regulatory organizations and standardization bodies are also putting considerable effort in introducing these bands to be used universally or at least in large part of the world to facilitate roaming, simplify device implementation and cost reduction. Nonetheless, the use of regional or country specific bands would still be inevitable. There is also an increasing demand and use of multi-mode user Equipments (UEs), which consist of multiple Radio Access Technology (RAT) and multiple bands per RAT. The UE typically constantly searches for all supported bands especially when roaming or when loosing coverage. This has the negative impact of draining UE power, increasing the average search delay and requiring UE hardware design to cater for worst case scenario increasing cost and implementation complexity.
Below various concepts and technological aspects relating to multi-mode UEs are outlined.
Frequency Bands and Channel Arrangement Principles
Standardized Frequency Bands in 3GPP
Below a list of frequency bands presently specified for different technologies: Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) Frequency Division Duplex (FDD) and Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network (E-UTRAN).
GSM Bands
The GSM bands are specified in Third Generation Partnership Project (3GPP) specification No. TS 45.005, “Radio Transmission and Reception”. The same specification provides a complete set of minimum radio requirements for GSM mobile terminal and the base stations. These requirements are used by the manufacturers to build products.                i) T-GSM 380 band:        ii) T-GSM 410 band:        iii) GSM 450 Band:        iv) GSM 480 Band;        v) GSM 710 Band:        vi) GSM 750 Band:        vii) T-GSM 810 Band:        viii) GSM 850 Band:        ix) Standard or primary GSM 900 Band, P-GSM:        x) Extended GSM 900 Band, E-GSM (includes Standard GSM 900 band):        xi) Railways GSM 900 Band, R-GSM (includes Standard and Extended GSM 900 Band);        xii) T-GSM 900 Band;        xiii) DCS 1 800 Band:        xiv) PCS 1 900 Band:UTRA FDD Bands        
All UTRA FDD (Wideband code division multiple access, WCDMA) frequency bands are specified in 3GPP specifications Nos. TS 25.101 and TS 25.104. The same set of specifications provides a complete set of UTRA FDD minimum radio requirements for mobile terminal and the base stations. These requirements are used by the manufacturers to build products.
TABLE 1UTRA FDD frequency bandsDL frequenciesOperatingUL FrequenciesUE receive, Node BBandUE transmit, Node B receivetransmitI1920-1980MHz2110-2170MHzII1850-1910MHz1930-1990MHzIII1710-1785MHz1805-1880MHzIV1710-1755MHz2110-2155MHzV824-849MHz869-894MHzVI830-840MHz875-885MHzVII2500-2570MHz2620-2690MHzVIII880-915MHz925-960MHzIX1749.9-1784.9MHz1844.9-1879.9MHzX1710-1770MHz2110-2170MHzXI1427.9-1452.9MHz1475.9-1500.9MHzXII698-716MHz728-746MHzXIII777-787MHz746-756MHzXIV788-798MHz758-768MHzE-UTRA Bands
All E-UTRA (FDD and Time Division Duplex, TDD) frequency bands are specified in 3GPP specifications Nos. TS 36.101 and TS 36.104. The same set of specifications provides a complete set of E-UTRA (FDD and TDD) minimum radio requirements for mobile terminal and the base stations. These requirements are used by the manufacturers to build E-UTRA products.
As shown in table 2, presently there are 25 E-UTRA bands and about 5 additional are being standardized. In the next few years they are expected to approach in the order of 50 bands as new spectrum is being freed. Additional spectrum would especially be required to introduce advanced E-UTRAN or the so-called Long Term Evolution (LTE) advanced. Compared to the legacy UEs, an E-UTRA or advanced E-UTRA UE is likely to support more bands to ensure universal operation. This will require that a typical E-UTRA UE on the average has to scan or search more frequencies than the legacy UEs.
TABLE 2E-UTRA frequency bandsUplink (UL)Downlink (DL)eNode B receiveeNode B transmitUE transmitUE receiveE-UTRA BandFUL—low-FUL—highFDL—low-FDL—highDuplex Mode 11920 MHz-1980 MHz2110 MHz-2170 MHzFDD 21850 MHz-1910 MHz1930 MHz-1990 MHzFDD 31710 MHz-1785 MHz1805 MHz-1880 MHzFDD 41710 MHz-1755 MHz2110 MHz-2155 MHzFDD 5824 MHz-849 MHz869 MHz-894 MHzFDD 6830 MHz-840 MHz875 MHz-885 MHzFDD 72500 MHz-2570 MHz2620 MHz-2690 MHzFDD 8880 MHz-915 MHz925 MHz-960 MHzFDD 91749.9 MHz-1784.9 MHz1844.9 MHz-1879.9 MHzFDD101710 MHz-1770 MHz2110 MHz-2170 MHzFDD111427.9 MHz-1452.9 MHz1475.9 MHz-1500.9 MHzFDD12698 MHz-716 MHz728 MHz-746 MHzFDD13777 MHz-787 MHz746 MHz-756 MHzFDD14788 MHz-798 MHz758 MHz-768 MHzFDD. . .17704 MHz-716 MHz734 MHz-746 MHzFDD. . .331900 MHz-1920 MHz1900 MHz-1920 MHzTDD342010 MHz-2025 MHz2010 MHz-2025 MHzTDD351850 MHz-1910 MHz1850 MHz-1910 MHzTDD361930 MHz-1990 MHz1930 MHz-1990 MHzTDD371910 MHz-1930 MHz1910 MHz-1930 MHzTDD382570 MHz-2620 MHz2570 MHz-2620 MHzTDD391880 MHz-1920 MHz1880 MHz-1920 MHzTDD402300 MHz-2400 MHz2300 MHz-2400 MHzTDDObservation about Frequency BandsBand Applicability: Single Versus Multiple Technologies
Some of these bands can be used for all or at least several technologies whereas some of them are limited to either fewer or only to one technology. For instance GSM band I (450 MHz) is not used for UTRAN or E-UTRAN. On the other hand GSM bands extended 800 (band V), 1800 (band VII) and 1900 (band VIII) can also be used by UTRAN (bands VIII, band III and band II) and E-UTRAN (bands 8, band 3 and band 2) respectively.
Regional Versus Universal Bands
3GPP specifications specify the channel arrangement, signaling and requirements for different bands, which can potentially be used in different countries or regions. This allows the mobile terminal and network manufacturers to built products according to the need and market demands in different parts of the world.
However it is up to each regional or national regulatory body to decide the bands to be used and the applicable technologies in their respective region or country. Since a band can be used for more than one technology, the band can potentially be split for different technologies. This split can vary from one country to another. For instance the UTRAN FDD band I and E-UTRAN band 1 is considered to be universal as it is widely available and allocated in large number of countries across the globe e.g. in Europe, Asia and Australia. However it can also be shared among different technologies and the actual split can vary from one country to another. For instance band 1, which comprises of 60 MHz in each direction, is presently split in Japan as follows:                First 20 MHz is allocated to one operator and is used for cdma2000 technologies        Central 20 MHz is allocated to another operator and is used for UTRAN FDD        Lateral 20 MHz is allocated to yet another operator and is used for UTRAN FDD        
This means it is of no use for a terminal operating in Japan to scan the entire UMTS FDD band I (i.e. all 276 possible raster positions).
Similarly UMTS FDD bands XII-XIV and the corresponding E-UTRA FDD bands 12-14 are only currently used in US. Furthermore, the entire band 12 and 13 are presently allocated to different operators.
Some of the bands are also purely regional as they are applicable or used solely in certain countries or group of countries or even in specific regions within a country.
An overview of regional organization responsible for allocating frequencies for different technologies within their respective region or country is provided below.
Regional Frequency Allocation
Above an overview of frequency bands specified in 3GPP standard is provided. This allows cellular network manufacturers to build products. However, as described above it is entirely up to the regional or even country wide regulatory or relevant authority to decide whether certain frequency band is allowed or not in their jurisdiction.
In USA, the Federal Communications Commissions (FCC) is responsible for attributing licenses for various Wireless Communications Service (WCS) including fixed, mobile, radiolocation or satellite services.
Similarly in Europe, Electronic Communications Committee (ECC), which is part of European conference of postal and telecommunications administrations (CEPT), is responsible for radio communications and telecommunications matters. More specifically European Radiocommunications Office (ERO) supports ECC in developing and maintaining the frequency allocation for CEPT member countries. As of today there are 48 CEPT member countries. Each member country has its own frequency allocation. However, the ERO allocation table is used as the basis for developing national frequency allocation. Similar regional organizations are active in other parts of the world for allocating frequencies in their region for different technologies.
In summary the actual bands and frequencies used in a particular region or a country is regulatory by the regional or country wide organizations responsible for frequency allocation in their respective regions.
Reallocation and Swapping of Bands
As the demand for particular a technology grows new frequency bands are standardized. There is also an increasing trend towards freeing spectrum used by conventional technologies and reallocating it to more modern technologies.
For instance frequency bands used for older technologies like advanced mobile phone system (AMPS) in USA and PDC in Japan are being standardized for WCDMA and LTE. One recent example is that of Japanese 800 MHz band (i.e. 815-830 MHz in UL), which is presently and exclusively used for cdma2000. But very recently it is being standardized for LTE, see RP-080884, “Extended UMTS/LTE 800” Approved work Item. This means multi-RAT UE supporting both cdma2000 and LTE and operating in this band (800 MHz) will have to search for both cdma2000 and LTE systems.
Multimode User Equipments
A multimode UE is a UE supporting more than one band per radio access technology (multi-band) and/or supporting more than one radio access technology (multi-RAT). This is further elaborated below:
Multi-Band UE
Most of the UE today support multiple bands even for the same technology. This is because either the service provider may own carriers in different bands and would like to make efficient use of carriers by performing load balancing on different carriers. Secondly multi-band UE enables roaming since many bands are regional. A well known example is that of multi-band GSM terminal with 800/900/1800/1900 bands ensuring almost universal operation (i.e. in US, Europe, Asia and other regions).
Multi-RAT User Equipments
In the past legacy UE typically supported one RAT e.g. only GSM between one to 3 bands. Presently multi-mode UE with two RATs e.g. GSM and WCDMA are quite common. In near future multimode UE implemented with 3 or more RAT will probably also be common.
According to E-UTRAN standard, see 3GPP TS 36.133, “Evolved Universal Terrestrial Radio Access (E UTRA); Requirements for support of radio resource management”, a multi-RAT UE may support any combination of one or more of the following technologies while fulfilling the inter-working requirements (e.g. cell reselection and handovers between RATs):                E-UTRAN FDD        E-UTRAN TDD        UTRAN FDD (WCDMA)        UTRAN TDD        GSM        cdma2000 1xRTT        HRPD.        
For example a UE with the following combination of RATs may exist in the future:                I E-UTRAN FDD, WCDMA, GSM        II E-UTRAN TDD, E-UTRAN TDD, GSM        III E-UTRAN FDD, E-UTRAN TDD, WCDMA, GSM        IV E-UTRAN FDD, E-UTRAN TDD, UTRAN TDD        V E-UTRAN FDD, cdma2000 1xRTT, HRPD        VI E-UTRAN FDD, E-UTRAN TDD, cdma2000 1xRTT, HRPD        
This is one example and several other cases may also exist. An increase in the number of RAT and bands per RAT results in that a UE will have to search for more frequencies increasing UE processing and power consumption.
Below some additional concepts are described namely: channel raster and channel numbering.
Channel Raster
In order to simplify the frequency search or the so-called initial cell search the center frequency of a radio channel is specified to be an integral multiple of well define, generally a fixed number, called channel raster. This enables a UE to tune its local oscillator only at one of the raster points assuming it to be the center frequency of the channel being searched.
The channel raster in WCDMA is 200 KHz but for certain channels and bands it is also 100 KHz. In LTE E the channel raster for all channels (i.e. all bandwidths) is 100 KHz. The channel raster directly impacts the channel numbering, which is described in the next section.
There is a trade-off between shorter and larger channel raster. If raster is too small then UE has to consider more hypotheses regarding the location of the center frequency of a channel when performing the frequency search. A guard band between adjacent channels within the same or between different operators may have to be introduced to reduce the adjacent channel interference or the effect of out of band emissions. Therefore too large raster would lead to wastage of frequency band due to the coarser resolution of guard bands.
Channel Numbering
A channel number is designated by an absolute radio frequency number. In GSM, WCDMA and E-UTRAN the channel numbers are called as Absolute Radio Frequency Channel Number (ARFCN), UTRA Absolute Radio Frequency Channel Number (UARFCN) and E-UTRA Absolute Radio Frequency Channel Number (EARFCN) respectively. In FDD systems separate channel numbers are specified for uplink and downlink. In TDD there is only one channel number since the same frequency is used in both directions.
The channel number for each band can be derived from the expressions and mapping tables defined in the relevant 3GPP specifications set out above. For initial frequency searching the UE has to search at all possible raster frequencies. However, for the UEs camped on the cell, the network signals the absolute channel number(s) for performing measurements, mobility decisions such as cell reselection or commanding handover to certain cell belonging to certain frequency channel of the same or of different RAT etc.
Cell Search Procedure
In most technologies like in GSM, UTRA or E-UTRA the UE searches cells in hierarchical manner and is called as hierarchical cell search procedure. This means UE typically acquires frequency synchronization, cell frame timing and cell physical identity in tandem. These concepts are discussed in the following sections.
Frequency Search or Band Scanning
When the UE is powered on, it first searches the list of all possible frequencies (or channels) in a frequency band. A multi-mode UE does this for all bands for RAT it supports unless explicitly forbidden.
The goal is to find in a particular band, the most suitable frequency channels in use in that region. In the next phase the UE proceeds with remaining task or more specifically acquires the cell timing and cell identity (ID) of neighbor cells, which are operated on the same frequency channel found in the first step. This is further described below. The process of searching frequency channel is often called as the initial cell search. However, band scanning and frequency search are also commonly used terms in literature. Here the term ‘frequency search’ is used.
To understand the complexity involved in frequency search procedure it can be helpful to for example consider UTRA FDD band I or E-UTRA band 1 (uplink: 1920-1980 MHz and downlink: 2110-2170 MHz).
Assume a UTRA FDD case where normally 12 bi-directional channels can be supported since each channel is of 5 MHz. Note that channels can also partly overlap in which case one or more extra channel might be possible to squeeze in. Alternatively if a guard band is used then fewer than 12 channels would be used. Considering 200 KHz channel raster in UTRA FDD, the uplink UARFCN and down UARFNC ranges, which are [9612, 9888] and [10562, 10838] respectively, can be derived from 3GPP specification TS 25.101. This means that a UE has to scan 276 possible channels when scanning UTRA FDD band I. Consider next the E-UTRA FDD case where channel bandwidth is variable ranging from 1.4 MHz to 20 MHz. The combination of relatively shorter channel raster and the variable channel bandwidth will cause a dramatic increase in the number of hypotheses an LTE UE will need to consider during frequency search or the so-called band scanning. To slightly facilitate the cell search procedure in LTE all synchronization sequences and physical broadcast signaling are sent over the central 1.4 MHz regardless of the channel bandwidth in use. At a later stage after the cell synchronization has been fully sought, the UE obtains the information about the actual channel bandwidth used in a cell by reading the physical broadcast channel (PBCH). Since E-UTRA FDD employs channel raster of 100 KHz, therefore for E-UTRA FDD band 1 the UE has to scan at least twice as many or more specifically 600 possible channels as specified in 3GPP specification TS 36.101.
This means that the complexity of the frequency search increases proportionally with the increase in the number of bands to scan. In other words the frequency search delay will linearly increase with the number of frequencies searched. As can be understood the complexity due to searching becomes manifold in case of a multi-mode terminal, which has to scan multiple bands for each supported RAT. Furthermore, the task of band/RAT scanning considerably drains UE battery especially since this is mostly done in idle mode.
The impact is significant when roaming to a visited network as described below or when a UE looses coverage of particular band or RAT.
The 3GPP standard does not specify any performance requirements for frequency search or the so-called initial cell search. Instead requirements are specified only for neighbor cell search discussed below. Similarly neither detailed frequency search nor neighbor cell search procedures are specified in the standard. Instead frequency arrangement and channel structure e.g. channel raster, synchronization sequences etc are designed to assist these procedures.
The identities of home public land mobile network (HPLMN) and the corresponding access technology with their priority are provided on the SIM card, see 3GPP specifications Nos. TS 23.122, “Technical Specification Group Core Network and Terminals; Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode” and TS 31.102, “Technical Specification Group Core Network and Terminals; Characteristics of the Universal Subscriber Identity Module (USIM) application”.
A PLMN is a set of an access network, core network and other necessary mobile network elements or entities forming a complete mobile network. Similarly the subscriber can manually select and set the priority order of PLMN and the corresponding access technology or RAT. Nonetheless UE needs to initially scan the frequency bands in order to ensure that it camps on the cell of the desired PLMN, e.g. one prioritized by operator where location area or tracking area update can be successfully performed.
Frequency Search Performance Results
As stated above there are no standardized performance requirements for frequency search. Here a summary of WCDMA and LIE frequency search performance results for band I based on some earlier analyses is described. According to the analysis provided in A. Nielsen and S. Korpela, “WCDMA Initial Cell Search” Proc. IEEE VTC 2000, the searching of entire UTRA FDD band I (all possible 276 channels) even in relatively strong radio conditions (e.g. SCH SINR=−14 dB) requires on the average 10 seconds. Note that UTRA FDD cell search needs to work down to synchronization channel (SCH) SINR=−20 dB. Around this lowest level the band I search takes an exorbitant average time of about 60 seconds.
In similar but more recent studies presented in 3GPP R4-072076, “LTE PLMN band scan strategy: performance discussion” NXP, Philips it has been shown that the UTRA FDD band I search can be in the order of 7 seconds in the worst case. In the same contribution it has been shown that even in the most favorable conditions the E-UTRA FDD band 1 search can be in the order of 70 seconds and in the worst case can be between 100-300 seconds.
Cell Timing and Cell ID Acquisition
During the frequency searching a UE generally also detects the timing of the strongest cell. This however depends upon the specific algorithm used for frequency search. For instance a UE typically performs correlation over the synchronization sequences while assuming a certain center frequency.
In any case after acquiring frequency synchronization the UE continues performing the neighbor cell search. It therefore continuously attempts to find the cell timing and physical ID of the cells operating on the acquired carrier frequency. Once a UE is camped on the strongest cell the broadcast information is downloaded and the location area update in UMTS or tracking area update in LTE is carried out. If authentication fails the UE attempts to connect to another suitable cell or to a cell of another allowed PLMN.
The 3GPP standard specifies the neighbor cell search requirements, which are defined in terms of time required to acquire cell ID (assuming carrier frequency is known) under certain side conditions e.g. under certain lowest synchronization channel (SCH) received or SCH signal to interference plus noise ration (SINR) levels. The maximum intra-frequency cell search delay requirements are 800 ms for both WCDMA, see 3GPP TS 25.133, “Requirements for support of radio resource management (FDD)” and E-UTRA, see 3GPP TS 36.133, “Evolved Universal Terrestrial Radio Access (E UTRA); Requirements for support of radio resource management”. The inter-frequency requirements scales with measurement gap density.
Impact of Roaming on Cell Search
During roaming a UE needs to scan all possible channels within all its supported bands. A home operator typically has a roaming agreement with multiple operators. Therefore, when being out of the home network a UE typically has a choice to camp on more than one visited network (i.e. visited PLMN). This further intensifies frequency search when the currently camped on cell becomes weaker. Similar behavior is observed when a UE looses coverage of its home network or the currently used band or RAT. That is why it can be noticed that a UE battery typically drains more quickly while roaming.
Satellite Based Mobile Positioning Methods
The popularity of a service allowing the determination of user positioning via a dedicated handheld device or an integrated mobile phone is on the rise. Furthermore, for safety purposes, mobile positioning is gradually becoming mandatory in several parts of the world. For instance, in the US the FCC mandate for Phase II E-911 services (emergency call public safety systems) in the near future will require that all mobile devices support positioning. Thus, in upcoming years most mobile devices are likely to support some sort of positioning mechanism.
Several methods are standardized and can be used for positioning (i.e. for determining mobile user position) in mobile communication. Some well known examples are: satellite based positioning, fingerprinting, time of arrival based method, etc.
Global Navigation Satellite System (GNSS) is a standard generic term for satellite navigation systems that enable subscriber to locate their position and acquire other relevant navigational information.
The global positioning system (GPS) and the European Galileo positioning system are well known examples of GNSS. Other potential systems, which are either proposed or those being developed are Russian GlObal Navigation Satellite System (GLONASS), Chinese COMPASS and Indian Regional Navigational Satellite System (IRNSS).
However, only GPS is currently in operation for more than a decade. The GPS comprises of a constellation of 24 to 32 medium earth orbit (MEQ) satellites revolving around the earth. They transmit pilot signals and other broadcast information, which are received and processed by the GPS receivers for determining geographical position. Signals from certain number of satellites (e.g. 5 or more) should be received in order for the GPS receiver to accurately location the geographical position of the user. Of course more number of visible satellites would further enhance the accuracy.
The assisted GNSS (A-GNSS) is a generic term used for any satellite based positioning method adapted for determining the position and other relevant information such as velocity of a mobile terminal.
The assisted GPS (A-GPS) is tailored to work with a user terminal (UE) enabling the UE subscribers to relatively accurately determine their location, time, and even velocity (including direction) in an open area environment provided sufficient number of GPS satellites are visible. A-GPS is standardized for UMTS, see 3GPP TS 25.171, “Requirements for support of Assisted Global Positioning System (A-GPS); Frequency Division Duplex (FDD)”. Similar work is underway for LTE, see RP-080995, “Positioning Support for LTE” Approved work Item.
The assisted A-GNSS or A-GPS requires that the cellular network (e.g. base station or radio network controller) provides data at least initially to assist an A-GPS capable mobile terminal to improve the measurement accuracy and to reduce the time required in determining mobile position. The assisted data includes reference time, visible satellite list, satellite signal Doppler, code phase, Doppler and code phase search windows, see 3GPP TS 25.171, “Requirements for support of Assisted Global Positioning System (A-GPS); Frequency Division Duplex (FDD)”. This information is obtained by a GPS receiver located at the serving base station, which can detect the navigational satellites more accurately. This data can be valid for a few minutes (e.g., less than 5 minutes) or longer depending on the code phase and Doppler search window size that can be accommodated by the UE.
Note that without network assisted data the UE can still determine its position. However in this case the accuracy of the position will be less accurate in some scenarios for example when fewer satellites are visible or when the signals received from the satellites are weak. The acquisition of the assisted data requires UE to first synchronize to the network i.e. camped on a cell. This means prior to camping on a cell the GPS based initial position of the UE without valid or up-to-date assisted data may incorporate a large error when received satellite signal(s) is weak. For instance the error in the position can be in order to hundreds of meters to 1 or 2 kilometers.
The A-GPS service evidently involves subscription charges, which can be considerable. Thus all GPS enabled cell phones may not use assisted GPS. Rather they can rely on the stand standalone non-assisted GPS, which is slower and relatively less accurate when satellite signals are weak, but is also free of charges. Indeed its performance is quite similar to that of a normal GPS receiver. Furthermore, in some implementation in the absence of the valid assisted data, an A-GPS receiver may also have an ability of falling back to a baseline non-assisted GPS receiver. For instance when UE is unable to detect a cell, such an A-GPS receiver may still be able to determine the subscriber position.
As is described above in existing solutions a UE when powered on performs frequency synchronization by making use of the stored information related to the last carrier frequencies used. The UE may also start searching frequencies which are used in the home network. A problem with such solutions is that the frequency bands and the carrier frequencies used in the visited network may be entirely different than those used in the home network or in the recently accessed network. The result is that in order to camp on to the appropriate carrier frequency, on the average, a conventional scheme will lead to a long frequency acquisition time. Repeated and long frequency synchronization time results in an increased UE power consumption and a shortened UE standby time.
Furthermore, the coverage of newly introduced technology (e.g. LTE) generally expands gradually. Therefore during considerable period of the network expansion phase the coverage is limited to certain parts of a country or a region. Initially the coverage will be patchy but will still be generally continuous in a given local area. Otherwise the network under deployment may not be practically usable by early subscribers. For instance an operator may initially cover major urban localities with this network. Subscribers moving in and out of this limited or local coverage areas or those residing at the border area will often loose coverage and frequently cause their UE (with this RAT capability) to search for the same RAT. However, the UE power can be saved and other existing RATs can be searched relatively quickly provided UE avoids searching those RATs, which are not deployed at the current location of the UE. A similar approach also applies for a newly introduced frequency band of an existing technology e.g. deploying WCDMA over a newly allocated band. The introduction of new band requires the upgrading of various parts of the network infrastructure and in some cases also requires the re-planning of the legacy network e.g. the co-sited deployment of 800 MHz and 2.6 GHz bands, which have very large difference in propagation loss. Thus coverage of new band may gradually expand and would impact UE frequency search performance and UE battery life.
An operator may also choose to deploy different technologies in different regions according to the demand of local subscribers and expected revenue returns. Assume only WCDMA is deployed in some very sparsely populated rural regions whereas both WCDMA and LTE are accessible in all urban and sub-urban areas. Presently, a typical UE supports only two RATs. But in near future the introduction of addition technologies (GSM, WCDMA, UTRA TDD, LTE FDD, LTE TDD, cdma2000 technologies) and large number of bands for each technology will significantly increase the demand of multi-mode UE (multi-RAT and multi-band per RAT). However, still only sub-set of combinations of RAT and bands are likely to be used by an operator in a particular region or country. Since in several cases the same band can be used for multiple RATs, therefore a particular operator, which has limited amount of spectrum, may be obliged to prioritize certain bands for certain RAT.
Radio Requirements
The radio requirements are specified for both UE and the BS and broadly categorize into transmitter and receiver requirements. The radio requirements are specified for different technologies (GSM, UTRA FDD and E-UTRA) for UE and BS in the following specifications:                i. 3GPP TS 45.005, “Radio Transmission and Reception”        ii. 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”.        iii. 3GPP TS 25.104, “Base station (BS) radio transmission and reception (FDD)”.        iv. 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); User Equipment (UE) radio transmission and reception”        v. 3GPP TS 36.104, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Base station (BS) radio transmission and reception”        
The radio requirements are briefly elaborated below:
Although a wireless device typically operates in a well defined portion of the frequency band, emissions outside its operating bandwidth and also outside its operating band are unavoidable. Therefore, UEs as well as base stations have to fulfill a specified set of out of band (OOB) emission requirements, which are important transmitter requirements. The objective of OOB emission requirements is to limit the interference caused by transmitters in the User Equipment and radio base station outside their respective operating bandwidths to the adjacent carriers or bands. In fact, all wireless communication standards (e.g. GSM, UTRAN, E-UTRAN, WLAN etc), clearly specify the OOB emission requirements to limit or at least minimize the unwanted emissions. They are primarily approved and set by the national and international regulatory bodies e.g. ITU-R, FCC, ARIB, ETSI etc.
The major OOB emission requirements, which are typically specified by the standards bodies and eventually enforced by the regulators in different countries and regions for both UE and the base stations comprise of:                Adjacent Channel Leakage Ratio (ACLR)        Spectrum Emission Mask (SEM)        Spurious emissions        Additional spurious emissions or additional maximum power reduction        In-band unwanted emissions        
The specific definition and the specified level of these requirements can vary from one system to another. Typically these requirements ensure that the emission levels outside an operating bandwidth or band in some cases remain several tens of dB below compared to the wanted signal in the operating bandwidth. Although OOB emission level tends to decay dramatically further away from an operating band but they are not completely eliminated at least in the adjacent carrier frequencies.
Since emissions are unavoidable therefore both the User equipment and radio base station should be capable of receiving the desired or the so-called wanted signal under the presence of reasonable interference due to emissions from other devices operating in adjacent carriers or bands. Hence a large number of radio receiver requirements are specified and several of them are also primarily set by the regional or international regulatory bodies. The objective of the radio receiver requirements is to ensure that the devices are able to adequately receive the desired signal in the presence of allowed out of band emissions from the other devices. Some examples of radio receiver requirements are:                Adjacent Channel Selectivity (ACS)        Blocking Characteristics        
There are few important observations about the radio requirements. Firstly different technologies may have different requirements. For instance in E-UTRAN the UE ACLR is 30 dB for an adjacent E-UTRA carrier. However, E-UTRA UE ACLR for an adjacent UTRA carrier is 3 dB tighter i.e. 33 dB. In UTRA FDD (WCDMA) the UE ACLR is 33 dB.
Secondly, the same technology may have different requirement level in different region. This particularly depends upon the type and nature of the technologies co-existing in a particular region. For example, the E-UTRA FDD operation of 10 MHz bandwidth in band 13 requires very stringent additional maximum power reduction requirement. However, the UTRA FDD, which uses only 5 MHz, operating in band 13 would be required to fulfill relatively modest maximum power reduction requirement.
Furthermore, for the same frequency band the radio requirements may differ from one region to another. This is also due to the type and nature of the technologies co-existing in a particular region.
In the patent application in EP 1739991 A1 a User Equipment (UE) determines its location via GPS and scans a selected set of frequencies rather than the entire band used in that particular area.
The solution described in EP 1739991 A1 is however only feasible if there are very few limited bands and RATs and the frequency information remains stable over longer time. However, over the past few years, the number of possible frequency bands and RATs has grown with rapid pace. As described above, several combinations of multimode UEs can be envisaged in the market. It is therefore not feasible to pre-code the complete set of frequency information of all supported RAT pertaining to all possible regions of the world. Secondly, the frequency information may change from one region to another. Though the modification may not be very frequent but is most relevant for the subscribers traveling to these destinations where the change has occurred. Thus, if there is any change in the frequency information, it may not be feasible to provide the complete set of updated information to all the subscribers. On the other hand it is important that subscribers traveling to such areas or frequent travelers are able to acquire up to date frequency information of these locations ahead of their journey.
Furthermore, EP 1739991 A1 is not concerned with frequency searching at the dedicated mobile networks (e.g. onboard mobile network, railways mobile network, on ships etc). This is because in order to improve the band scanning in these dedicated networks, the signaled information described in EP 1739991 A1 comprising of frequencies and regional coordinates is not sufficient.
Hence, there exist a need for a method and a device that is able to provide improved searching of frequencies in a cellular radio system.